1. Technical Field
The present invention relates to document illumination devices, contact-type image sensor modules, and image reading devices.
2. Related Art
Contact-type image sensor modules have been widely used in image reading devices such as image scanners, facsimile devices, and so on for some time. With a contact-type image sensor module, an image sensor whose width is greater than or equal to the width of a document to be read is disposed in close proximity to the document, and an imaging optical device that forms an erect unmagnified image is disposed between the document and the image sensor. For the imaging optical device, it is common to use a lens array in which many graded index lenses are arranged.
In recent years, attempts have been made to reduce the cost of manufacturing such imaging optical devices by using a combination of a lens member in which many curved-surface lenses are formed and a light-blocking member in which many through-holes are formed in correspondence with the individual curved-surface lenses, instead of a graded index lens (for example, see JP-A-2000-214305). Generally speaking, such a lens member, in which curved-surface lenses are formed, is several times thicker than a graded index lens (that is, the lens radii are greater).
In the case where a contact-type image sensor module is used in the image reading device, the document illumination device is disposed extremely close to the lens member and a region where the surface of the document intersects with the optical axes of the lenses is illuminated. Such a document illumination device includes a rod-shaped light-conducting member that is longer in the direction in which the lens array is arranged and a light source disposed near an end surface of the light-conducting member in the lengthwise direction thereof.
However, a lens member in which curved-surface lenses are formed is several times thicker than a graded index lens, and thus there is a long distance between the optical axes of the lenses and the light-conducting member; as a result, there is a problem in that the region of the document to be scanned becomes dark.